Database Constraints

A constraint in a database is a rule that ensures the accuracy and integrity of data. Primary key, Unique, and Foreign key are all constraints in relational databases. A Candidate key, on the other hand, is a set of attributes that could uniquely identify a record but is not necessarily used as a constraint until selected as a primary key. Therefore, "Candidate key" is not a constraint in the sense that it does not impose a rule on the data unless it is chosen to be the primary key.